A bouncing concrete foundation in a farm implement shed has been found to be the reason for an anomalously high ground-shaking recording made during the magnitude 7.8 Kaikōura earthquake in November 2016.

A seismic instrument at Waiau in North Canterbury, near to the epicentre, recorded a peak ground acceleration of 3.2g, or just over three times the force of gravity. This was twice as high as the next highest ground-shaking measurement made during the earthquake.

Dr Yoshi Kaneko

Scientists and engineers use the force of gravity as a measure of the strength of ground-shaking during earthquakes. The recordings have a number of applications including earthquake hazard assessment and guiding engineering standards.

Scientists thought right away that the Waiau recording was suspect. Following an exhaustive review, scientists from New Zealand and Japan have found that the actual ground-shaking at the Waiau site was only about 1.5g.

In a paper published this week in the journal Scientific Reports, the scientists say the Waiau instrument was firmly attached to a concrete slab in a farm implement shed.

“There was nothing obvious about the way the instrument was installed or the site that would suggest that it might be prone to recording anything other than the actual strength of shaking at ground level,’’ said joint author Yoshihiro Kaneko, a seismologist at GNS Science.

“Our investigation showed that the slab foundation of the building actually detached from the ground during the earthquake and bounced several times. As the instrument was attached to the slab, it produced a recording that did not accurately reflect the ground shaking of the earthquake. We call this ‘the flapping effect’.”

Dr Kaneko said the seismic equipment was working correctly. However, it was recording the movements of the bouncing slab rather than the ground itself.

When the instrument was installed it would have been hard to foresee that the slab foundation would behave in this way during a large earthquake, he said.

“This effect only becomes evident in earthquakes that are larger than magnitude 6 and they don’t happen very often.”

Dr Kaneko said scientists were in the process of reviewing recordings made by other seismic instruments during large earthquakes to check for the occurrence of similar phenomena.

“This is an active area of research and this is the way science advances. Large earthquakes invariably throw up new questions and new information that helps in the overall understanding of earthquakes and their impacts.”